Thermosetting, high-solids coating composition and method of forming topcoat by using the same

ABSTRACT

The present invention provides a thermosetting high solids coating composition comprising (A) a carboxyl-containing compound, (B) a polyepoxide, and (C) a copolymer prepared by polymerizing monomer components comprising (a) 30 to 50% by weight of vinyltrimethoxysilane and/or vinyltriethoxysilane, (b) 5 to 15% by weight of N-methylol(meth)acrylamide alkyl ether, and (c) 35 to 65% by weight of another polymerizable unsaturated monomer, and a method for forming a topcoat using said composition. The composition of the present invention is highly effective in forming a coating film excellent in resistance to both acids and scratch, and is superior in storage stability and recoat adhesion.

TECHNICAL FIELD

The present invention relates to a novel thermosetting, high solidscoating composition and method of forming topcoat by using the same.

BACKGROUND ART

Thermosetting topcoat compositions comprising a hydroxyl-containingacrylic resin and a melamine resin have been heretofore chiefly used forcoating automotive exterior panels. However, in recent years, acid rainhas posed a worldwide problem of etching and blots or like stains oncoating films. With the widespread use of car washers, scratches made oncoating films by car washers have presented another problem. In thissituation, there is a need for topcoat compositions, particularly clearcoat compositions, which are capable of forming coating filmssatisfactory in both acid resistance and scratch resistance.

Usually, the scratch resistance of a coating film can be improved byincreasing the crosslinking density of the film. The acid resistance ofa coating film can be improved by incorporating an acid resistantcrosslinking system into the film. However, a method has been scarcelyproposed for giving both acid resistance and scratch resistance to acoating film.

For example, Japanese Unexamined Patent Publication No. 222,753/1990discloses that a coating film having high crosslinking density, and thushaving high scratch resistance, can be obtained by baking an acrylicresin of high hydroxyl value in the presence of a monomeric melamineresin and an acid catalyst. However, this crosslinking system has adrawback of poor acid resistance since the coating film crosslinked by amelamine resin readily decomposes when contacted with an acid.

Methods have been proposed for imparting both acid resistance andscratch resistance to a coating film by incorporating an acid-resistantcrosslinking system to a less acid-resistant melamine resin crosslinkingsystem. The proposed systems include, for example, a compositecrosslinking system having a combination of carboxyl group/epoxygroup/hydroxyl group/melamine resin (Japanese Unexamined PatentPublication No. 247,264/1990), a composite crossllnking system having acombination of hydroxyl group/alkoxysilyl group/melamine resin(WO91/16,383), etc. However, these systems have not satisfactorilyachieved significant improvements in acid resistance because of thepresence of a melamine resin.

Melamine resin-free crosslinking systems have also been proposed. Theproposals include a crosslinking system having only a combination ofcarboxyl group/epoxy group or carboxyl group/epoxy group/hydroxyl group(e.g., Japanese Unexamined Patent Publications Nos. 87,288/1987,45,577/1990 and 287,650/1991). Yet, these systems have a shortcomingthat the resulting coating films, although superior in acid resistance,are inferior in scratch resistance owing to the low crosslinking densityof the cured films.

A resin composition comprising a carboxyl-containing acrylic polymer andan epoxy- and hydrolyzable silyl-containing compound is known as acrosslinking system having a combination of carboxyl group/epoxygroup/hydrolyzable silyl group (Japanese Unexamined Patent PublicationNo. 187,749/1987), but this composition has a defect that the curingreaction of the polymer with the compound is unsatisfactory because ofthe steric hindrance caused by the presence of the epoxy group andhydrolyzable silyl group in the same molecule.

A resin composition comprising a hydroxyl- and carboxyl-containingsilicone polymer, a carboxyl- and carboxylic acid ester group-containingpolymer and a hydroxyl- and epoxy-containing polymer has been proposedas a crosslinking system having a combination of carboxyl group/epoxygroup/hydroxyl group which contains the silicone polymer as a base resin(Japanese Unexamined Patent Publication No. 166,741/1994). However, thecured coating of this composition is defective in that it is not alwaysfully satisfactory in crosslinking density and is poor in the recoatadhesion which is one of the important characteristics of coatingcompositions for automotive exterior panels.

On the other hand, it is of urgent necessity in the field of coatingcompositions to take measures for the control on the use of organicsolvents, from the viewpoints of prevention of air pollution andconservation of resources. As one of such measures, there is a demandfor the development of high solids coating compositions which contain aless amount of solvents and have a higher solid concentration.

DISCLOSURE OF INVENTION

An object of the present invention is to provide a novel thermosettinghigh solids coating composition free of the foregoing prior artdrawbacks, and a method of forming a topcoat using said composition.

Another object of the invention is to provide a novel thermosetting highsolids coating composition which is capable of forming a coating filmexcellent in both acid resistance and scratch resistance, and a methodfor forming a topcoat using said composition.

A further object of the invention is to provide a novel thermosettinghigh solids coating composition excellent in storage stability andrecoat adhesion, and a method for forming a topcoat using saidcomposition.

Other objects and features of the invention will become apparent fromthe following description.

The present invention provides a thermosetting high solids coatingcomposition comprising:

(A) a carboxyl-containing compound,

(B) a polyepoxide, and

(C) a copolymer prepared by polymerizing monomer components comprising(a) 30 to 50% by weight of vlnyltrimethoxysilane and/orvinyltriethoxysilane, (b) 5 to 15% by weight ofN-methylol(meth)acrylamide alkyl ether and (c) 35 to 65% by weight ofanother polymerizable unsaturated monomer.

Further, the present invention provides a method for forming a topcoatcomprising the step of successively forming a colored base coat and aclear coat on a substrate, said colored base coat and/or clear coatbeing formed from the above coating composition.

The inventors of the present invention conducted extensive research toachieve the above objects and found that the foregoing objects can beachieved by a thermosetting high solids coating composition comprising acarboxyl-containing compound (A), a polyepoxide (B) and theabove-specified copolymer (C). The present invention has been completedbased on this novel finding.

The high solids coating composition and the method for forming a topcoataccording to the invention will be described below in further details.

The high solids coating composition of the invention essentiallycomprises (A) the carboxyl-containing compound, (B) the polyepoxide and(C) the specific copolymer containing at least one hydrolyzablealkoxysilyl group selected from methoxysilyl and ethoxysilyl, and atleast one alkyl-etherified N-methylol group.

The carboxyl-containing compound (A) for use in the invention contains acarboxyl group in its molecule and usually has an acid value of 50 to500 mg KOH/g, preferably 80 to 300 mg KOH/g.

An acid value of less than 50 mg KOH/g of the compound (A) is likely tolower the curability of the resulting composition and to thereby reducethe acid resistance, scratch resistance and stain resistance of thecoating film. On the other hand, an acid value of more than 500 mg KOH/gtends to decrease the compatibility with the polyepoxide (B) and thecopolymer (C) and to thereby reduce the storage stability of thecomposition. Hence an acid value of the compound (A) outside said rangeis undesirable.

The following compounds (A-1) to (A-4) are usable as the compound (A).

(A-1): a polymer having, in its molecule, a group formed by halfesterification of an acid anhydride group

The group formed by half esterification of an acid anhydride groupconsists of a carboxyl group and a carboxylic acid ester group, and isobtained by subjecting an acid anhydride group and an aliphaticmonohydric alcohol to addition reaction for ring opening, namely halfesterification. Hereinafter the group may be referred to simply as “halfester group”.

The compound (A-1) can be easily prepared by copolymerizing anunsaturated monomer having a half ester group and other polymerizableunsaturated monomers in the conventional manner, or by the samecopolymerization with the exception of using an unsaturated monomerhaving an acid anhydride group in place of the unsaturated monomerhaving a half ester group, followed by half esterification of the acidanhydride group.

Useful unsaturated monomers having an acid anhydride group are, forexample, a maleic anhydride, an itaconic anhydride or the like. Usefulunsaturated monomers having a half ester group are, for example, thoseprepared by half esterification of the acid anhydride group in theunsaturated monomer.

The half esterification can be done either before or after thecopolymerization reaction, as described above. Examples of aliphaticmonohydric alcohols useful in the half esterification arelow-molecular-weight monohydric alcohols such as methanol, ethanol,isopropanol, t-butanol, isobutanol, methyl cellosolve, ethyl cellosolve,etc. The half esterification reaction is conducted in the conventionalmanner at a temperature ranging from room temperature to about 80° C.,if necessary in the presence of tertiary amine serving as a catalyst.

Examples of other polymerizable unsaturated monomers arehydroxyl-containing unsaturated monomers; (meth)acrylic acid esters;vinyl ethers and aryl ethers; olefin compounds and dlene compounds;hydrocarbon ring-containing unsaturated monomers; nitrogen-containingunsaturated monomers; acrylic monomers containing hydrolyzablealkoxysilyl group; etc.

Examples of the hydroxyl-containing unsaturated monomers are C₂₋₈hydroxyalkyl esters of acrylic or methacrylic acids, such as2-hydroxyethyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate,hydroxybutyl (meth)acrylate, etc.; monoesters of (meth)acrylic acid orlike unsaturated carboxylic acids with polyethylene glycol,polypropylene glycol, polybutylene glycol or like polyether polyols;monoethers of 2-hydroxyethyl (meth)acrylate or like hydroxyalkyl estersof (meth) acrylic acid with polyethylene glycol, polypropylene glycol,polybutylene glycol or like polyether polyols; monoesterificationproducts or diesterification products of maleic anhydride, itaconicanhydride or like acid anhydride group-containing unsaturated compoundswith ethylene glycol, 1,6-hexanediol, neopentyl glycol or like glycols;hydroxyethyl vinyl ether or like hydroxyalkyl vinyl ethers; allylalcohols and the like; 2-hydroxypropyl (meth)acrylate; adducts ofα,β-unsaturated carboxylic acid with “CARDULA E10” (product of ShellPetrochemical Co., Ltd., trade name), α-olefin epoxide or like monoepoxycompounds; and adducts of glycidyl (meth)acrylate with acetic acid,propionic acid, p-tert-butylbenzoic acid, aliphatic acid or likemonobasic acids; adducts of the above hydroxyl-containing unsaturatedmonomers with lactones (e.g., ε-caprolactone, γ-valerolactone, etc.);and the like.

Examples of (meth)acrylic acid esters are C₁₋₂₄ alkyl esters orcycloalkyl esters of acrylic or ethacrylic acids, such as methylacrylate, ethyl acrylate, ropyl acrylate, isopropyl acrylate, n-butylacrylate, isobutyl acrylate, t-butyl acrylate, hexyl acrylate,2-ethylhexyl acrylate, n-octyl acrylate, decyl acrylate, stearylacrylate, lauryl acrylate, cyclohexyl acrylate, methyl methacrylate,ethyl methacrylate, n-propyl methacrylate, isopropyl methacrylate,n-butyl methacrylate, isobutyl methacrylate, t-butyl methacrylate, hexylmethacrylate, 2-ethylhexyl methacrylate, octyl methacrylate, decylmethacrylate, lauryl methacrylate, stearyl methacrylate, cyclohexylmethacrylate, etc.; C₂₋₁₈ alkoxyalkyl esters of acrylic or methacrylicacids, such as methoxybutyl acrylate, methoxybutyl methacrylate,methoxyethyl acrylate, methoxyethyl methacrylate, ethoxybutyl acrylate,ethoxybutyl methacrylate, etc.; and the like.

Examples of vinyl ethers and aryl ethers are ethyl vinyl ether, n-propylvinyl ether, isopropyl vinyl ether, butyl vinyl ether, t-butyl vinylether, pentyl vinyl ether, hexyl vinyl ether, octyl vinyl ether and likechain-like alkyl vinyl ethers; cyclopentyl vinyl ether, cyclohexyl vinylether and like cycloalkyl vinyl ethers; phenyl vinyl ether, trivinylether and like aryl vinyl ethers; benzyl vinyl ether, phenethyl vinylether and like aralkyl vinyl ethers; allyl ethyl ether and like allylethers; etc.

Examples of olefin compounds and diene compounds are ethylene,propylene, butylene, vinyl chloride, butadiene, isoprene, chloroprene,etc.

Examples of hydrocarbon ring-containing unsaturated monomers arestyrene, α-methylstyrene, phenyl (meth)acrylate, phenylethyl(meth)acrylate, phenylpropyl (meth)acrylate, benzyl (meth)acrylate,phenoxyethyl (meth)acrylate, cyclohexyl (meth)acrylate,2-acryloyloxy-ethylhydrogen phthalate, 2-acryloyloxypropylhydrogenphthalate, 2-acryloyloxypropylhexahydrohydrogen phthalate,2-acryloyloxypropyltetrahydrohydrogen phthalate, ester ofp-t-butyl-benzoic acid with hydroxyethyl (meth)acrylate, dicyclopentenyl(meth)acrylate, etc.

Examples of nitrogen-containing unsaturated monomers arenitrogen-containing alkyl (meth)acrylates such as N,N-dimethylaminoethyl(meth)acrylate, N,N-diethylaminoethyl (meth)acrylate,N-t-butylaminoethyl (meth)acrylate, etc.; polymerizable amides such asacrylamide, methacrylamide, N-methyl (meth)acrylamide, N-ethyl(meth)acrylamide, N,N-dimethyl (meth)acrylamide, N,N-dimethylaminopropyl(meth)acrylamide, N,N-dimethylaminoethyl (meth)acrylamide, etc.;aromatic nitrogen-containing monomers such as 2-vinylpyridine,1-vinyl-2-pyrrolidone, 4-vinylpyridine, etc.; polymerizable nitrilessuch as acrylonitrile, methacrylonitrile, etc.; allylamines; and so on.

Examples of the hydrolyzable alkoxysilyl-containing acrylic monomer areγ-(meth)acryloyloxypropyl-trimethoxysilane,γ-(meth)acryloyloxypropylmethyldimethoxysilane,β-(meth)acryloyloxyethyltrimethoxysilane,γ-(meth)acryloyloxypropyltriethoxysilane,γ-(meth)acryloyloxypropylmethyldiethoxysilane and the like.

The copolymerization can be carried out by conventional methods forpolymerization of unsaturated monomers. The most suitable method issolution type radical polymerization carried out in an organic solvent,in view of application range and costs. Stated more specifically, thedesired polymer can be easily obtained by copolymerization at atemperature of about 60 to about 150° C. in an organic solvent in thepresence of a polymerization initiator such as azo catalysts, peroxidecatalysts or the like. Useful organic solvents include aromatic solventssuch as xylene and toluene, ketone solvents such as methyl ethyl ketoneand methyl isobutyl ketone, ester solvents such as ethyl acetate, butylacetate, isobutyl acetate and 3-methoxybutyl acetate, and alcohols suchas n-butanol and isopropyl alcohol, etc.

The half ester group- or acid anhydride group-containing unsaturatedmonomer and other polymerizable unsaturated monomer are copolymerized inthe following proportions based on the total weight of the monomers. Theproportion of the half ester group- or acid anhydride group-containingunsaturated monomer is about 5 to about 40% by weight, preferably about10 to about 30% by weight, in view of curability and storage stability.The proportion of the other polymerizable unsaturated monomer is about60 to about 95% by weight, preferably about 70 to about 90% by weight.Of the other polymerizable unsaturated monomers, styrene is usedsuitably in a proportion of up to about 20% by weight in view ofweatherability of cured coating. When an acid anhydride group-containingunsaturated monomer is used, half esterification is carried out aftercopolymerization as described above.

The compound (A-1) is preferably an acrylic polymer having a numberaverage molecular weight of 2,000 to 20,000. A number average molecularweight of less than 2,000 tends to impair the weatherability of thecured coating, whereas a number average molecular weight exceeding20,000 tends to reduce the compatibility with the polyepoxide (B) andthe copolymer (C). Hence a number average molecular weight of thecompound (A-1) outside said range is undesirable.

(A-2): polymer having a carboxyl group in its molecule

The number average molecular weight of the compound (A-2) is preferablyabout 2,000 to about 20,000. A number average molecular weight of lessthan 2,000 tends to impair the weatherability of the cured coating,whereas a number average molecular weight exceeding 20,000 tends toreduce the compatibility with the polyepoxide (B) and the copolymer (C).Hence a number average molecular weight of the compound (A-2) outsidesaid range is undesirable.

The compound (A-2) can be easily prepared by copolymerizing acarboxyl-containing unsaturated monomer and other polymerizableunsaturated monomers in the similar conventional manner for the abovecompound (A-1).

Examples of carboxyl-containing unsaturated monomers include acrylicacid, methacrylic acid, crotonic acid, itaconic acid, maleic acid,fumaric acid, 2-carboxyethyl (meth)acrylate, 2-carboxypropyl(meth)acrylate and 5-carboxypentyl (meth)acrylate.

Examples of other polymerizable unsaturated monomers include those usedin the preparation of the compound (A-1) such as (meth)acrylic acidesters; vinyl ethers and aryl ethers; olefin compounds and dienecompounds; hydrocarbon ring-containing unsaturated monomers andnitrogen-containing unsaturated monomers.

(A-3): carboxyl-containing polyester polymer

The number average molecular weight of the carboxyl-containing polyesterpolymer is not limited specifically, but preferably about 1,500 to about20,000.

The carboxyl-containing polyester polymer can be easily prepared bycondensation reaction of a polyhydric alcohol with a polycarboxylicacid. Useful polyhydric alcohols include, for example, ethylene glycol,butylene glycol, 1,6-hexanediol, trimethylolpropane, pentaerythritol,etc. Useful polycarboxylic acids include, for example, adipic acid,terephthalic acid, isophthalic acid, phthalic anhydride,hexahydrophthalic anhydride, etc. Stated more specifically, thecarboxyl-containing polyester polymer can be prepared, for example, by aone-step reaction using a reaction system containing excess carboxylgroup of a polycarboxylic acid, or by a reaction using a reaction systemcontaining excess hydroxyl group of a polyhydric alcohol to give ahydroxyl-terminated polyester polymer, followed by an addition reactionof the polymer with an acid anhydride group-containing compound such asphthalic anhydride, hexahydrophthalic anhydride, succinic anhydride,etc.

(A-4): A half ester formed by a reaction of polyol with 1,2-acidanhydride

The number average molecular weight of said half ester is not limitedspecifically, but usually as low as about 400 to about 1,000. Said halfester is highly reactive with an epoxy group and useful for forming ahigh solids coating composition.

Said half ester is obtained by reacting a polyol with 1,2-acid anhydrideunder conditions which can effect the ring-opening reaction of the acidanhydride but substantially do not cause polyesterification reaction.The product of such reaction has a low molecular weight and narrowmolecular weight distribution. Further, the product has a low volatileorganic content when contained in a composition and imparts excellentacid resistance and other properties to the resulting coating film.

The half ester is prepared by the reaction of a polyol with 1,2-acidanhydride in an inert atmosphere, such as nitrogen atmosphere, in thepresence of a solvent. Preferred solvent are ketones such as methyl amylketone, diisobutyl ketone, methyl isobutyl ketone and the like; aromatichydrocarbons such as toluene, xylene and the like; and other organicsolvents such as dimethylformamide, N-methylpyrrolidone and the like.

The reaction is carried out preferably at a low temperature of about150° C. or less. Specifically stated, the reaction temperature ispreferably about 70 to about 150° C., more preferably about 90 to about120° C. A temperature exceeding 150° C. causes polyesterificationreaction, whereas a temperature less than 70° C. results inunsatisfactory reaction rate. Hence, reaction temperatures outside thespecified range are not desirable.

Basically, the reaction time slightly varies depending on the reactiontemperature, but is usually about 10 minutes to about 24 hours.

For obtaining the desired half ester in the maximum yield, theequivalent ratio of the acid anhydride to the polyol is about 0.8:1 toabout 1.2:1, when calculating the acid anhydride as monofunctionalcompound.

The acid anhydride for use in the preparation of the desired half esterhas about 2 to about 30 carbon atoms excluding the carbon atoms in theacid moiety. Examples of such acid anhydrides are aliphatic acidanhydrides, alicyclic acid anhydrides, olefin acid anhydrides and cyclicolefin acid anhydrides and aromatic acid anhydrides. These acidanhydrides may have substituents insofar as the substituents do notadversely affect the reactivity of the acid anhydride or the propertiesof the resulting half ester. Examples of the substituents are chloro,alkyl and alkoxy groups. Examples of the acid anhydrides are succinicanhydride, methylsuccinic anhydride, dodecenylsuccinic anhydride,octadecenylsuccinic anhydride, phthalic anhydride, tetrahydrophthalicanhydride, methyltetrahydrophthalic anhydride, hexahydrophthalicanhydride, alkylhexahydrophthalic anhydride (such asmethylhexahydrophthalic anhydride), tetrafluorophthalic anhydride,endomethylenetetrahydrophthalic anhydride, chlorendic anhydride,itaconic anhydride, citraconic anhydride and maleic anhydride.

Usable polyols are those having about 2 to about 20 carbon atoms.Preferably usable are diols, triols, mixtures thereof, and like polyolshaving 2 to 10 carbon atoms. Preferable examples are aliphatic polyolssuch as ethylene glycol, 1,2-propanediol, 1,3-propanediol,1,4-butanediol, 1,5-pentanediol, glycerol, 1,2,3-butane triol,1,6-hexanediol, neopentyl glycol, diethylene glycol, dipropylene glycol,1,4-cyclohexane dimethanol, 3-methyl-1,5-pentanediol,trimethylolpropane, 2,2,4-trimethyl-pentane-1,3-diol, pentaerythritoland 1,2,3,4-butanetetraol. Aromatic polyols such as bisphenol A and bis(hydroxymethyl)xylene are also usable.

The polyepoxide (B) for use in the composition of the present inventionis a compound having an epoxy group in its molecule. The polyepoxideusually contains 0.8 to 15 mmol/g, preferably 1.2 to 10 mmol/g of theepoxy group.

An epoxy content of less than 0.8 mmol/g of the polyepoxide (B) islikely to lower the curability of the resulting composition and tothereby reduce the acid resistance, scratch resistance and stainresistance of the coating film, whereas an epoxy content of higher than15 mmol/g tends to decrease the compatibility with thecarboxyl-containing compound (A) and the copolymer (C) and to therebyreduce the storage stability of the composition. Hence, an epoxy contentof the polyepoxide (B) outside said range is undesirable.

Examples of the polyepoxide (B) include epoxy-containing acrylicpolymers; alicyclic epoxy-containing acrylic polymers; diglycidyl ether,2-glycidylphenyl glycidyl ether, 2,6-diglycidylphenyl glycidyl ether andlike glycidyl ether compounds; vinylcyclohexene dioxide, limonenedioxide and like glycidyl- or alicyclic epoxy-containing compounds;dicyclopentadiene dioxide, bis(2,3-epoxycyclopentyl)ether,epoxycyclohexenecarboxylic acid ethylene glycol diester,bls(3,4-epoxycyclohexyl methyl)adipate,3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexanecarboxylate,3,4-epoxy-6-methylcyclohexylmethyl-3,4-epoxy-6-methylcyclohexanecarboxylate and like alicyclic epoxy-containing compounds. Thesepolyepoxides may be used in combination.

Among these polyepoxides, preferably used are the epoxy-containingacrylic polymers and alicyclic epoxy-containing acrylic polymers havinga number average molecular weight of 2,000 to 20,000. A number averagemolecular weight of less than 2,000 is likely to lower theweatherability of the cured coating film, whereas a number averagemolecular weight of more than 20,000 tends to decrease the compatibilitywith the compound (A) and the copolymer (C). Hence, a number averagemolecular weight outside said range is undesirable.

Said epoxy-containing acrylic polymer or alicyclic epoxy-containingacrylic polymer is easily prepared by copolymerizing an epoxy-containingunsaturated monomer or alicyclic epoxy-containing unsaturated monomerwith other polymerizable unsaturated monomer in the same conventionalmanner for the aforementioned compound (A-1).

Examples of the epoxy-containing unsaturated monomer are glycidyl(meth)acrylate, allyl glycidyl ether and the like. Examples of thealicyclic epoxy-containing unsaturated monomer are3,4-epoxycyclohexylmethyl (meth)acrylate and the like.

Examples of other polymerizable unsaturated monomers are those mentionedas the polymerizable unsaturated monomers for the above-mentionedcompound (A-1), i.e., hydroxyl-containing unsaturated monomers;(meth)acrylic acid esters; vinyl ethers and aryl ethers; olefincompounds and diene compounds; hydrocarbon ring-containing unsaturatedmonomers; and nitrogen-containing unsaturated monomers; acrylic monomerscontaining hydrolyzable alkoxysilyl group; etc.

The copolymer (C) for use in the composition of the present invention isa copolymer prepared by polymerizing 100% by weight of a monomercomponent consisting of (a) 30 to 50% by weight of vinyltrimethoxysilaneand/or vinyltriethoxysilane, (b) 5 to 15% by weight ofN-methylol(meth)acrylamide alkyl ether and (c) 35 to 65% by weight ofanother polymerizable unsaturated monomer. When the proportion of themonomer (a) is lower than 30% by weight, the composition using theresulting copolymer has lower curability and reduced scratch resistanceof the coating film. On the other hand, when the proportion of themonomer (a) is higher than 50% by weight, the composition tends to lowerrecoat adhesion. In addition, when the proportion of the monomer (b) islower than 5% by weight, the recoat adhesion of the composition islowered, while the coating film is likely to cause yellowing when theproportion is higher than 15% by weight.

Said copolymer (C) usually has the content of hydrolyzable alkoxysilylgroup, i.e., methoxysilyl group and/or ethoxysilyl group, of 0.5 to 4.0mmol/g, preferably 1.0 to 3.5 mmol/g, and a number average molecularweight of 1,000 to 5,000, preferably 1,200 to 4,000. The content of theabove hydrolyzable alkoxysilyl group of less than 0.5 mmol/g is likelyto lower the curability of the resulting coating composition and reducethe acid resistance and scratch resistance of the resulting coatingfilm, whereas the content of the hydrolyzable alkoxysilyl group of morethan 4.0 mmol/g tends to decrease the compatibility with the compound(A) and the polyepoxide (B). Hence, the content of the hydrolyzablealkoxysilyl group outside said range is undesirable. When the copolymer(C) has a number average molecular weight of less than 1,000, thecurability of the coating composition is lowered and the acid resistanceof the resulting coating film is degraded. On the other hand, when thecopolymer (C) has a number average molecular weight of more than 5,000,the compatibility with the compound (A) and the polyepoxide (B) islowered and it becomes difficult to obtain the high solids coatingcomposition. Thus, the number average molecular weight outside saidrange is undesirable.

Said copolymer (C) can be readily prepared by copolymerizing the abovemonomer component consisting of the monomers (a), (b) and (c) by thesame conventional procedure for preparing the compound (A-1).

The monomer (a) is vinyltrimethoxysilane, vinyltriethoxysilane or amixture of vinyltrimethoxysilane and vinyltriethoxysilane in anyproportion.

The monomer (b) is a N-methylol(meth)acrylamide alkyl ether, of whichalkyl moiety preferably has 1 to 4 carbon atoms. Particularly preferredare N-methylolacrylamide butyl ether, N-methylolacrylamide methyl ether,etc.

Examples of other polymerizable unsaturated monomers, that is, themonomer (c), are those mentioned as other polymerizable unsaturatedmonomers for the above compound (A-1), namely, (meth)acrylic acidesters; hydroxyl-containing unsaturated monomers; vinyl ethers and arylethers; olefin compounds and diene compounds; hydrocarbonring-containing unsaturated monomers; and nitrogen-containingunsaturated monomers; hydrolyzable alkoxysilyl-containing acrylicmonomers; etc.

In copolymerization for preparing the copolymer (C) of the invention, itis preferable to use a C₁₋₂₄ alkyl ester or cycloalkyl ester of acrylicor methacrylic acid as the polymerizable unsaturated monomer (c). It ismore preferable to copolymerize a C₁₋₄ alkyl ester of acrylic ormethacrylic acid as the monomer (c).

The thermosetting high solids coating composition of the inventioncontains, as essential components, the carboxyl-containing compound (A),the polyepoxide (B) and the copolymer (C) containing a specifichydrolyzable alkoxysilyl group, preferably in the following proportions.The proportion of the compound (A) and the polyepoxide (B) is preferablysuch that the equivalent ratio of the carboxyl group in the compound (A)to the epoxy group in the polyepoxide (B) is 1:0.5 to 0.5:1, theproportion of the copolymer (C) is preferably 3 to 200 parts by weightper 100 parts by weight of the combined amount of the compound (A) andthe polyepoxide (B). Proportions outside said range tend to reduce thecurability and thereby lower the acid resistance and scratch resistanceof the coating film, and thus are not preferable. More preferably, theproportions of the compound (A) and the polyepoxide (B) are such thatthe equivalent ratio of the carboxyl group in the former to the epoxygroup in the latter is 1:0.7 to 0.7 to 1, and the proportion of thecopolymer (C) is 9 to 100 parts by weight per 100 parts by weight of thecombined amount of the compound (A) and the polyepoxide (B).

The coating composition of the present invention may contain curingcatalysts when necessary.

Usable curing catalysts include those which are effective for thering-opening esterification of the carboxyl group in the compound (A)with the epoxy group in the epoxide (B), such as tetraethylammoniumbromide, tetrabutylammonium bromide, tetraethylammonium chloride,tetrabutylphosphonium bromide, triphenylbenzylphosphonium chloride andlike quaternary salt catalysts; triethylamine, tributylamine and likeamines; etc. Among them, quaternary salt catalysts are preferable.Catalysts consisting of said quaternary salt and an acidic phosphoricacid compound (e.g., dibutylphosphoric acid, etc.) in equivalent amountare also preferable, since such catalysts improve the storage stabilityof the coating composition and prevent the lowering of the spray coatingamenability owing to the reduction of the electric resistance of thecoating composition, without impairing the above-mentioned catalyticaction.

Also usable are curing catalysts effective for the hydrolyticcondensation of the methoxysilyl or ethoxysilyl group in the copolymer(C). Examples of such catalysts include acids, for example, sulfuricacid, phosphoric acid and like inorganic acid, p-toluenesulfonic acid,dodecylbenzenesulfonic acid, dinonylnaphthalene-sulfonic acid,dinonylnaphthalenedisulfonic acid, trichloracetic acid,trifluoromethanesulfonic acid and like organic acids; monoethanolamine,diethanolamine, triethylamine, tributylamine, 2-aminomethylpropanol andlike amine compounds; neutralized products of said acids and said aminecompounds; dibutyltin dilaurate, dibutyltin diacetate and like tincatalysts; tetrabutyl titanate and like titanium catalysts; etc.

When the above curing catalyst is used, the proportion is usually about0.01 to about 5 parts by weight per 100 parts by weight of the combinedamount of the carboxyl-containing compound (A), polyepoxide (B) andhydrolyzable alkoxysilyl-containing copolymer (C), calculated as solids.

When necessary, the composition of the invention may contain dehydratingagents such as trimethyl orthoacetate for preventing the coatingcomposition from degradation caused by moisture in the air or thesolvent.

Further, generally known pigments such as coloring pigments, extenderpigments, anti-corrosive pigments and the like can be added to thecoating composition of the present invention, when necessary.

Examples of useful coloring pigments are organic pigments such asquinacridone red and like quinacridone pigments, pigment red and likeazo pigments, phthalo-cyanine blue, phthalocyanine green, perylene redand like phthalocyanine pigments; inorganic pigments such as titaniumoxide and carbon black; metallic pigments such as aluminum flake, nickelflake, copper flake, brass flake, chrome flake, pearl mica and coloredpearl mica.

The composition of the invention may optionally contain resins such aspolyester resins, alkyd resins, silicone resins, fluorine resins, andnonaqueous particulate polymer etc., and may also contain a minor amountof melamine resins, blocked isocyanate or like crosslinking agents.Further, the composition of the invention may contain conventionaladditives for coating compositions such as UV absorbers, oxidationinhibitors, surface modifiers, defoaming agents, etc.

The thermosetting coating composition of the invention is usually usedin the form of an organic solvent type coating composition. Examples ofuseful organic solvents include those for coating compositions such asaromatic or aliphatic hydrocarbon solvents; alcohol solvents; estersolvents; ketone solvents; ether solvents, etc. Organic solvents used inpreparing the components of the composition can be used as it is, oradditional organic solvents may be used.

The coating composition of the present invention may be prepared as ahigh solids composition. The solid concentration of said composition isusually as high as about 40 to about 80% by weight, preferably about 45to about 75% by weight.

The thermosetting coating composition of the invention can be applied tovarious substrates by conventional coating methods and can be fullycured by heating at a temperature of about 100 to about 180° C. forabout 10 to about 60 minutes to give a coating film having high acidresistance, scratch resistance and stain resistance. It is presumed thatthe composition of the present invention is fully cured by crosslinkingbased on the ring-opening esterification reaction of the carboxyl groupin the compound (A) and the epoxy group in the epoxide (B), as well ason the hydrolytic self-condensation reaction of methoxysilyl groupand/or ethoxysilyl group in the copolymer (C).

The method for forming a topcoat according to the present inventioncomprises the step of successively forming a colored base coat and clearcoat on a substrate, said colored base coat and/or clear coat beingformed from the thermosetting coating composition of the invention.

The substrates to be used for forming the topcoat include steel panelstreated by chemical conversion, electrophoretically coated with aprimer, and optionally coated with an intercoat; various plasticsubstrates optionally surface-treated and optionally coated either witha primer or with a primer and an intercoat; substrates produced by acomposite of the steel and plastics, etc.

In the method of forming a topcoat according to the invention, thethermosetting coating composition of the invention is used as at leastone of the composition for forming the colored base coat and thecomposition for forming the clear coat.

The thermosetting coating composition of the invention is capable offorming a coating film excellent especially in resistance to acids andscratch and is, therefore, preferably used as a resin component of acoating composition for forming a clear coat.

The thermosetting coating composition of the invention as such can beused as a clear coat composition. The clear coat composition may containcoloring pigments in an amount which will not completely hide thecolored base coat.

The thermosetting coating composition of the invention can be used as acolored base coat composition by properly incorporating therein theorganic pigments, inorganic pigments, metallic pigments and likecoloring pigments mentioned above.

The method of forming a topcoat according to the invention isparticularly suitable for forming a topcoat on automotive exteriorpanels. The coating methods of the invention include those known in theautomotive industry, for example, those comprising forming a coloredbase coat and a clear coat by 2-coat 1-bake system or 2-coat 2-bakesystem, and those comprising forming a colored base coat, a clear coatand a clear coat by 3-coat 1-bake system or 3-coat 2-bake system.

The most desirable method of forming a topcoat according to theinvention comprises the steps of applying a composition for forming acolored base coat to a substrate, applying a composition for forming aclear coat to the uncured surface of the base coat, and curing the twocoats by heating according to 2-coat 1-bake system.

Described below is the coating method by 2-coat 1-bake system using thethermosetting coating composition of the invention as the resincomponent of a clear coat composition.

In 2-coat 1-bake system, a colored base coat composition is firstapplied to the above-mentioned substrate by conventional coating methodssuch as spray coating or the like.

The colored base coat composition may be the thermosetting coatingcomposition of the invention, or may be of the type per se known andconventionally used in a method for forming a topcoat.

Examples of colored base coat compositions per se known include, forexample, those comprising a curable resin component and said coloringpigment. Useful curable resin components are, for example, acrylicresin/amino resin mixture, alkyd resin/amino resin mixture, polyesterresin/amino resin mixture, acrylic resin/polyisocyanate mixture, alkydresin/polyisocyanate mixture, polyester resin/polyisocyanate mixture,etc. The amino resins used in said mixtures include melamine resins. Theform of these known colored base coat compositions is not specificallylimited and can be a desirable type selected from organic solvent type,nonaqueous dispersion type, aqueous dispersion type, aqueous solutiontype, high solids type, etc.

Application devices for spray coating include those conventionally used,such as an air spray gun, airless spray gun, air spray typeelectrostatic coater, airless spray type electrostatic coater, rotaryatomization type electrostatic coater, etc.

The colored base coat composition is applied preferably to a thicknessof about 10 to about 30 μm (when cured). The colored base coat thusdeposited is left to stand at room temperature for a few minutes or isforce-dried at a temperature of about 50 to about 80° C. for a fewminutes and then the thermosetting coating composition of the inventionis applied as a clear coat composition.

The clear coat composition can be applied by the same manner using thesame application device as in the application of the colored base coatcomposition.

The clear coat composition is applied preferably to a thickness of about20 to about 80 μm (when cured).

Both the colored base coat and the clear coat thus applied areconcurrently cured by heating at about 100 to about 180° C. for about 10to about 60 minutes.

In the method for forming a topcoat according to the present invention,the composition for forming the colored base coat is preferably anaqueous type, or an organic solvent type having a solid concentration ofabout 35 to about 65% by weight (when applied by spray coating). Thecomposition for forming the clear coat is preferably of an organicsolvent type having a solid concentration of about 45 to about 75% byweight (when applied by spray coating).

The articles coated by the coating method of the invention have acoating film excellent in resistance to acids and scratch, and thecoating film thus formed is particularly suitable for coveringautomotive exterior panels.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention is described below in more detail with referenceto Examples wherein the parts and percentages are all by weight.

PREPARATION EXAMPLE 1

Preparation of acrylic polymer (a-1) having carboxyl group

A 5-liter glass flask equipped with a stirrer, thermometer and condenserwas charged with 553 parts of xylene and 276 parts of 3-methoxybutylacetate. The mixture was heated to 125° C. by an electrothermic mantle.At the same temperature, a mixture of the following monomer componentswas added dropwise at a uniform rate over a period of 4 hours.p-Tert-butylperoxy-2-ethyl hexanoate is a polymerization initiator.

n-Butyl methacrylate 432 parts Isobutyl methacrylate 346 parts Laurylmethacrylate 360 parts Styrene 72 parts Methacrylic acid 86 partsAcrylic acid 144 parts p-Tert-butylperoxy-2-ethylhexanoate 72 parts

The mixture was aged for 30 minutes. Added dropwise was a mixture of 277parts of 3-methoxybutyl acetate and 14.4 parts ofp-tert-butylperoxy-2-ethyl hexanoate over a period of 2 hours. Themixture was aged for 2 hours, giving a solution of a carboxyl-containingacrylic polymer (a-1) of a final conversion of 100%.

The obtained polymer solution had a polymer solid content of 70% and aGardner viscosity (25° C.) of V. The polymer had a number averagemolecular weight of 3,000 and an acid value of 117 mg KOH/g.

PREPARATION EXAMPLE 2

Preparation of half ester (a-2) formed by addition reaction of polyolwith 1,2-acid anhydride

A 5-liter glass flask equipped with a stirrer, thermometer and condenserwas charged with 236 parts of 3-methyl-1,5-pentanediol, 134 parts oftrimethylolpropane, 1078 parts of hexahydrophthalic anhydride and 780parts of xylene. The mixture was heated to 120° C. in a nitrogenatmosphere to initiate a reaction. The reaction mixture was maintainedat the same temperature for 4 hours and then cooled, giving a solutionof a half ester (a-2) having a solid content of 65% and a Gardnerviscosity (25° C.) of R. The half ester had an acid value of 271 mgKOH/g.

PREPARATION EXAMPLE 3

Preparation of acrylic polymer (b-1) having epoxy group

A 5-liter glass flask equipped with a stirrer, thermometer and condenserwas charged with 410 parts of xylene and 77 parts of n-butanol. Themixture was heated to 125° C. by an electrothermic mantle. At the sametemperature, a mixture of the following monomer components was addeddropwise at a uniform rate over a period of 4 hours.Azobisisobutyronitrile is a polymerization initiator.

Glycidyl methacrylate 432 parts 2-Hydroxyethyl acrylate 216 partsn-Butyl acrylate 504 parts Styrene 288 parts Azobisisobutyronitrile 72parts

The mixture was aged for 30 minutes. Added dropwise was a mixture of 90parts of xylene, 40 parts of n-butanol and 14.4 parts ofazobisisobutyronitrile over a period of 2 hours. The mixture was agedfor 2 hours, giving a solution of an epoxy-containing acrylic polymer(b-1).

The obtained polymer solution had a polymer solid content of 70% and aGardner viscosity (25° C.) of W. The polymer had a number averagemolecular weight of 3,000 and an epoxy content of 2.11 mmol/g.

PREPARATION EXAMPLE 4

Alicyclic epoxy-containing acrylic polymer (b-2)

A solution of alicyclic epoxy-containing acrylic polymer (b-2) wasprepared in the same manner as in Preparation Example 3 with theexception of changing only the monomer composition as follows.

3,4-Epoxycyclohexylmethyl methacrylate 576 parts 2-Hydroxyethylmethacrylate 216 parts Styrene 288 parts n-Butyl acrylate 360 parts

The obtained polymer solution had a polymer solid content of 70% and aGardner viscosity (25° C.) of Y.

The polymer had a number average molecular weight of 3,000 and an epoxycontent of 2.04 mmol/g.

PREPARATION EXAMPLE 5

Preparation of copolymer (c-1) having methoxysilyl group andalkyl-etherified N-methylol group

A 5-liter glass flask equipped with a stirrer, thermometer and condenserwas charged with 300 parts of xylene, 500 parts of butyl acetate and 200parts of n-butanol. The mixture was heated to 120° C. by anelectrothermic mantle. At the same temperature, a mixture of thefollowing monomer components was added dropwise at a uniform rate over aperiod of 4 hours.

Vinyltrimethoxysilane 400 parts Ethyl acrylate 500 partsN-methylolacrylamide butyl ether 100 parts t-Butylperoxyisopropylcarbonate 20 parts

The mixture was aged for 30 minutes. Added dropwise was 10 parts oft-butylperoxyisopropyl carbonate over a period of 2 hours, followed byaging for 1 hour. The solvent was removed under reduced pressure toconcentrate the solution until the resin solid concentration increasedto 70%, thus giving a solution of a copolymer (c-1). The solution had aGardner viscosity of A (at 25° C.). The obtained copolymer (c-1) had anumber average molecular weight of 1,500 and methoxysilyl group contentof 2.7 mmol/g.

PREPARATION EXAMPLE 6

Preparation of organic solvent type metallic base coat composition (Y-1)

(1) Preparation of hydroxyl-containing acrylic resin

In a mixed solvent of xylene/n-butanol (70/30 by weight) was polymerizeda mixture of 30 parts of methyl methacrylate, 59 parts of ethylacrylate, 10 parts of 2-hydroxyethyl acrylate and 1 part of acrylic acidat 100° C., using azobisisobutyronitrile as a polymerization initiator.A solution of hydroxyl-containing acrylic resin having a resin solidcontent of 50% was obtained. The resin had a number average molecularweight of 25,000.

(2) Preparation of the coating composition (Y-1)

50% solution of hydroxyl-containing 110 parts acrylic resin obtainedabove 88% “CYMEL 370” 28 parts 20% solution of CAB 100 parts Aluminumpaste 20 parts

A mixture of the above components was adjusted to a viscosity of 13seconds (Fordcup #4/20° C.) with a solvent mixture consisting of 30parts of toluene, 20 parts of isobutyl alcohol, 30 parts of cellosolveacetate and 20 parts of “SWASOL 1000” (product of Cosmo Oil Co., Ltd.,trade name, hydrocarbon solvent), giving an organic solvent type coatingcomposition (Y-1) having a nonvolatile content of about 40%.

The above-mentioned 88% ° “CYMEL 370” (trade name, product ofMitsui-Cyanamid, Ltd.) is a partially methyl-etherified melamine resinhaving a resin solid content of 88% in isopropanol. The 20% solution ofCAB is a 20% solution of cellulose acetate butylate in a mixed solventof toluene/n-butyl acetate (50/50 by weight). The aluminum paste was“Aluminum Paste #55-519” (product of Toyo Aluminum Co., Ltd., tradename, metallic pigment).

PREPARATION EXAMPLE 7

Preparation of an aqueous metallic base coat composition (M-1)

(1) Preparation of an aqueous dispersion of acrylic resin (W-1)

A reactor was charged with 140 parts of deionized water, 2.5 parts of30% “NEWCOL 707SF” (trade name, product of Nihon Nyukazai Co., Ltd.,surfactant) and 1 part of the monomer mixture (i) shown below. Themixture was stirred in a nitrogen stream. At 60° C., a monomer emulsionconsisting of 4 parts of 3% ammonium persulfate and 42 parts ofdeionized water was placed dropwise into the reactor over a period of 4hours using a metering pump. After the addition, the mixture was agedfor 1 hour.

Monomer mixture (i)

Methyl methacrylate 55 parts Styrene 10 parts n-Butyl acrylate 9 parts2-Hydroxyethyl acrylate 5 parts Methacrylic acid 1 part

Then, 20.5 parts of the monomer mixture (ii) shown below and 4 parts of3% ammonium persulfate were concurrently placed dropwise into thereactor at 80° C. over a period of 1.5 hours. After the addition, themixture was aged for 1 hour and filtered at 30° C. through a 200-meshnylon cloth filter. Deionized water was added, and the mixture wasadjusted to pH 7.5 with dimethylamino-ethanol, giving an aqueousdispersion of an acrylic resin (W-1) having an average particle diameterof 0.1 μm, a glass transition temperature (Tg) of 46° C. and anonvolatile content of 20%.

Monomer mixture (ii)

Methyl methacrylate 5 parts n-Butyl acrylate 7 parts 2-Ethylhexylacrylate 5 parts Methacrylic acid 3 parts 30% “Newcol 707SF” 0.5 part

(2) Preparation of an aqueous solution of acrylic resin (W-2)

A reactor was charged with 60 parts of butyl cellosolve and 15 parts ofisobutyl alcohol. The mixture was heated to 115° C. in a nitrogenstream. At a temperature of 115° C., there was added a mixture of 26parts of n-butyl acrylate, 47 parts of methyl methacrylate, 10 parts ofstyrene, 10 parts of 2-hydroxyethyl methacrylate, 6 parts of acrylicacid and 1 part of azobisisobutyronitrile over a period of 3 hours.After the addition, the mixture was aged at 115° C. for 30 minutes. Amixture of 1 part of azobisisobutyronitrile and 115 parts of butylcellosolve was added dropwise over a period of 1 hour, followed by agingfor 30 minutes. The mixture was filtered at 50° C. through a 200-meshnylon cloth filter. The obtained reaction product had an acid value of48 mg KOH/g, a viscosity (Gardner bubble viscometer) of Z4, anonvolatile content of 55% and a Tg of 45° C. The product was subjectedto equivalent neutralization using dimethylaminoethanol. Then deionizedwater was added, giving an aqueous solution of an acrylic resin (W-2)having a nonvolatile content of 50%.

(3) Preparation of an aqueous metallic base coat composition (M-1)

Aqueous dispersion of acrylic resin (W-1) 275 parts Aqueous solution ofacrylic resin (W-2) 40 parts “CYMEL 350” (trade name, product 25 partsof Mitsui Toatsu Chemicals Inc., fully methyl-etherified melamine resin)“ALUMINUM PASTE AW-500B” (trade name, 20 parts product of Asahi ChemicalMetals Co., Ltd., metallic pigment) Butyl cellosolve 20 parts Deionizedwater 253 parts

“THIXOL K-130B” (trade name, product of Kyoeisha Yushi Kagaku Kogyo KK,thickener) was added to the mixture of the above components to adjustthe mixture to a viscosity of 3,000 cps as measured with a Brookfieldviscometer (rotor revolution speed 6 rpm), giving an aqueous metalliccoating composition (M-1) having a nonvolatile content of about 19%.

EXAMPLES 1-5 AND COMPARATIVE EXAMPLES 1-3

A solution of mixed resins having the composition (part by weight onsolid basis) shown in Table 1 was prepared. Added thereto were 1 part of“TINUVIN 900” (trade name, product of Ciba-Geigy, ultraviolet absorber)and 0.1 part of “BYK-300” (trade name, product of BYK-Chemie Co.,surface modifier). The mixture was diluted with “SWASOL 1000” (tradename, product of Cosmo Oil Co., Ltd., hydrocarbon solvent) foradjustment to a viscosity of 30 seconds (Ford cup #4/20° C.), wherebythe coating composition of the present invention or a comparativecoating composition was prepared. Table 1 also shows the solid content(%) of the obtained coating compositions (when applied).

The compositions thus obtained were tested for storage stability by thefollowing method.

Storage stability: Each composition was diluted with “SWASOL 1000” to aviscosity of 30 seconds (Ford cup #4/20° C.). A 300 g portion of thediluted composition was placed into a beaker which was then looselycovered with aluminum foil. The composition was stored at 40° C. for 2weeks, and its viscosity (Ford cup #4/20° C.) was measured to check thedegree of thickening. The composition was evaluated by the followingcriteria:

A: viscosity less than 50 seconds (good storage stability), B: viscosityless than 70 seconds (slightly lower storage stability), and C:viscosity more than 70 seconds (poor storage stability).

The test results are shown in Table 1.

TABLE 1 Comparative Example Example Component 1 2 3 4 5 1 2 3 Compound(A) a-1 40 40 50 a-2 24 24 21 30 30 Polyepoxide (B) b-1 40 56 49 50 70b-2 40 56 70 Copolymer (C) c-1 20 20 20 20 30 Curing catalyst TBAB 0.50.5 0.5 0.5 0.5 0.5 0.5 0.5 “Nacure 4054” 0.5 0.5 0.5 0.5 0.5 0.5 0.50.5 Solid content 53 58 52 56 62 38 45 40 (%) Storage A A A A A A A Astability

In Table 1, TBAB, which is used as a curing catalyst, istetrabutylammonium bromide (product of Nippon Fine Chemical Co., Ltd.).“Nacure 4054” (product of King Industries, Inc., trade name) is a 50%solution of acidic phosphoric acid ester.

EXAMPLES 6-10 AND COMPARATIVE EXAMPLES 4-8

Topcoats were formed by the following method according to 2-coat 1-bakesystem using the coating compositions obtained in Examples 1 to 5 andComparative Examples 1 to 3 as clear coat compositions.

A dull steel panel of 0.8 mm thickness treated by chemical conversionwith zinc phosphate was coated with an epoxy-based cationicelectrodepositable coating composition to give a coating film of about20 μm thickness (when dried). The coated panel was baked at 170° C. for20 minutes, polished with sand paper (#400), and degreased by wipingwith petroleum benzine. The coated panel was further coated by air spraycoating with an automotive intercoating surfacer to give a coating filmof about 25 μm thickness (when dried). The coated panel was baked at140° C. for 30 minutes, subjected to wet rubbing with sand paper (#400),dehydrated for drying, and degreased by wiping with petroleum benzine,giving a test substrate.

The metallic base coat compositions obtained in Preparation Examples 6and 7 were applied to the test substrate to a thickness of 20 μm (whencured). The substrates coated with the coating composition (Y-1) wereleft to stand at room temperature for 5 minutes, whereas those coatedwith the coating composition (M-1) were force-dried at 80° C. for 10minutes. Each clear coat composition prepared above was applied to thecoated test substrate to give a coating film of 40 μm thickness (whencured). The coated panel was heated at 140° C. for 30 minutes, wherebythe two coats were cured to form a topcoat.

The topcoats thus formed were tested for properties as follows.

Film appearance

The metallic effect (brilliancy, whiteness and the like) was visuallyevaluated on A to C scale: A: excellent in metallic effect, B: poor inmetallic effect, and C: exceedingly poor in metallic effect.

Acid resistance

A half area of the coated panel was immersed in a 40% solution ofsulfuric acid. Then, the coated panel was left to stand at 50° C. for 5hours, followed by washing with water. The surface of coated panel wasvisually inspected and rated on A to C scale:

A: no change, B: substantially no change in the coating surface but aslight difference in film thickness between the immersed portion andunimmersed portion, and C: blushing on the coating surface.

Scratch resistance

An automobile with the coated panel attached to the roof was washed 15times in a car washer and the surface of coated panel was visuallyinspected. The car washer was a product of Yasui Sangyo Co., Ltd.available under the trade name “PO 20F WRC”. The results were evaluatedon A to C scale: A: substantially no scratch mark was found, B: slightscratch marks were found but to a negligible extent, and C: noticeablescratch marks were found.

Impact resistance

The coated panel was tested with a Du Pont impact tester using a weightof 500 g with a tip 0.5 inch in radius. The results were rated in termsof the maximum height (5 cm calibration) at which no cracking was causedby the weight dropped onto the coated panel.

Water resistance

The coated panel was dipped in warm water maintained at 40° C. for 240hours and washed with water, followed by visual inspection of thecoating surface. The evaluation was made on A to C scale: A: no change,B: slight dulling on the coating surface, and C: blushing on the coatingsurface.

Recoat adhesion

A first topcoat was formed following the above method according to2-coat 1-bake system. Then the topcoating compositions (base coatcomposition and clear coat composition) used in the first topcoat wereapplied to the coated panel in the same manner. Thereafter the coatedpanel was baked at 120° C. for 30 minutes to make a second topcoat. Thecoated panel was cut crosswise to reach the substrate, giving 100squares with a spacing of 1 mm. An adhesive tape was applied to and thenpeeled from the cut surface. The adhesion between the first and thesecond topcoats was rated on A to C scale: A: no peeling, B: slightlypeeled and C: considerably peeled.

The test results are shown in Table 2.

TABLE 2 Example 6 7 8 9 10 Base coat Y-1 Y-1 Y-1 M-1 M-1 compositionClear coat Example Example Example Example Example composition 1 2 5 1 5Property Film A A A A A appearance Acid A A A A A resistance Scratch A AA A A resistance Impact 50 50 50 50 50 resistance Water A A A A Aresistance Recoat A A A A A adhesion Comparative Example 4 5 6 7 8 Basecoat Y-1 Y-1 Y-1 M-1 M-1 composition Clear coat Comp. Comp. Comp. Comp.Comp. composition Example Example Example Example Example 1 2 3 1 3Property Film A A A A A appearance Acid A B A A A resistance Scratch C CC C C resistance Impact 40 40 40 40 40 resistance Water A A A A Aresistance Recoat C A A C A adhesion

According to the present invention, there is provided a novelthermosetting high solids coating composition which can form a coatingfilm excellent in both acid resistance and scratch resistance, and whichis superior in storage stability and recoat adhesion. Further providedis a method of forming a topcoat using said composition.

What is claimed is:
 1. A thermosetting high solids coating compositioncomprising: (A) a carboxyl-containing compound, (B) a polyepoxide, and(C) a copolymer prepared by polymerizing monomer components comprising(a) 30 to 50% by weight of vinyltrimethoxysilane and/orvinyltriethoxysilane, (b) 5 to 15% by weight ofN-methylol(meth)acrylamide alkyl ether, and (c) 35 to 65% by weight ofanother polymerizable unsaturated monomer.
 2. The composition accordingto claim 1 which is of an organic solvent and has a solids concentrationof about 40 to about 80% by weight.
 3. The composition according toclaim 1 wherein the compound (A) has an acid value of 50 to 500 mgKOH/g.
 4. The composition according to claim 1 wherein the compound (A)is an acrylic polymer having a number average molecular weight of 2,000to 20,000.
 5. The composition according to claim 1 wherein the compound(A) is a half ester formed by the addition reaction of polyol with1,2-acid anhydride.
 6. The composition according to claim 1 wherein thepolyepoxide (B) has an epoxy content of 0.8 to 15 mmol/g.
 7. Thecomposition according to claim 1 wherein the polyepoxide (B) is anacrylic polymer having a number average molecular weight of 2,000 to20,000.
 8. The composition according to claim 1 wherein the copolymer(C) has a methoxysilyl and/or ethoxysilyl content of 0.5 to 4.0 mmol/g.9. The composition according to claim 1 wherein the copolymer (C) has anumber average molecular weight of 1,000 to 5,000.
 10. The compositionaccording to claim 1 wherein the polymerizable unsaturated monomer (c)in the copolymer (C) is a C₁₋₂₄ alkyl ester or a cycloalkyl ester ofacrylic acid or methacrylic acid.
 11. The composition according to claim1 wherein the proportion of the compound (A) and the polyepoxide (B) issuch that the equivalent ratio of the carboxyl group in the compound (A)to the epoxy group in the polyepoxide (B) is 1:0.5 to 0.5:1, and theproportion of the copolymer (C) is 3 to 200 parts by weight per 100parts by weight of the combined amount of the compound (A) and thepolyepoxide (B).
 12. The composition according to claim 1, thecomposition comprising a curing catalyst.
 13. A method for forming atopcoat comprising the step of successively forming a colored base coatand a clear coat on a substrate, said colored base coat and/or clearcoat being formed from the composition of claim
 1. 14. The method forforming a topcoat according to claim 13, the method comprising the stepsof applying the composition for forming the colored base coat to asubstrate, applying the composition for forming the clear coat to theuncured surface of the base coat, and curing the two coats by heatingaccording to 2-coat 1-bake system.
 15. The method for forming a topcoataccording to claim 13 wherein the composition for forming the clear coatis a coating composition comprising: (A) a carboxyl-containing compound,(B) a polyepoxide, and (C) a copolymer prepared by polymerizing monomercomponents comprising (a) 30 to 50% by weight of vinyltrimethoxysilaneand/or vinyltriethoxysilane, (b) 5 to 15% by weight ofN-methylol(meth)acrylamide alkyl ether, and (c) 35 to 65% by weight ofanother polymerizable unsaturated monomer.
 16. An article coated by themethod according to claim 13.